One of the greatest uncertainties in the design of treatment plans for patients undergoing radiation therapy is the exact location and extent of non-unit density structures in the patient, and the correction of dose distributions for perturbations introduced by these non-unit density tissues in the vicinity of the treatment region. Without improved handling of these uncertainties, the criterion for satisfactory treatment (i.e., the dose delivered to the tumor must be accurate within 5 percent) cannot be attained for many patients for whom curative therapy is sought. This is particularly true for electron beam radiotherapy. To address these uncertainties, a concerted effort to apply recent technological innovations to radiation therapy treatment planning is proposed. These innovations include x-ray computed tomography. Its application to radiation therapy treatment planning represents an extension of our current efforts to utilize modern diagnostic tools for radiation therapy patients. The objective of this application is to achieve dose distributions accurate to within 2-3 percent. This objective requires development of new 3-dimensional approaches to dose computations especially for electron, but also for high-energy photon, beams in which the contributions of scattered radiation to the dose at any location within the patient are considered separately from the contributions of primary radiation. The validity of these approaches and the accuracy of computed doses corrected for non-unit density inhomogeneities will be verified experimentally as part of the proposed effort.